Sample Question - Sample Answers
Here is one question from last year's final exam. The exam was 3 hours long and this was
one of five question. Each question was worth 20% of the exam grade.
What follows are several answers given by the students. They have not been altered. The
grades they were assigned reflect the correctness of the answers and the amount of detail
presented.
Question:
What are the theories concerning the major evolutionary steps in the progression from
protochordate ancestors to the first vertebrates?
Answer #1 - assigned a score of 10/20:
The major evolutionary steps in the progression from protochordate ancestors to the first
vertebrates can be characterized by the appearance of:
1. dorsal tubular nerve cord
- comes from the neural plate in neural ectoderm through neurogenesis
- gives rise to neural crest cells which are necessary for further development of the
nervous system in higher vertebrates (such as neurons and ganglia) for complex
information processing
2. notochord
- comes from mesoderm
- provides the first form of body support in vertebrates (runs the entire length of the
animal)
- later, the notochord is replaced by the vertebral column
3. post anal tail
- if retained, the tail is the main provider of physical movement through thrust
- later, adaptations of this tail provide more complex locomotion (cantilever
system)
4. pharyngeal gill slits
- initially are used for ciliary feeding but later becomes adapted for respiration (gill
slits containing site of gas exchange in sharks)
- supporting gill slit adaptations evlove for more complex feeding in higher
vertebrates (jaw suspension)
5. segmentation and muscles
- plays a large role in establishing adaptations necessary for later forms of complex
articulation, affecting feeding and locomotory abilities
- later specializations allow vertebrates to be designed to survive best in a particular
niche
Bilateral symmetry (although already quite common) also established the need for
concentrating a sensory system at the head (which “meets” the environment first)
Answer #2 - assigned a score of 14/20
Protochordates
Chordates (Vertebrates)
The protochordates were mainly sessile, small filter feeders which used their skin
for respiration and captured food particles from the current with cilia to feed. The first
step towards vertebrates was the evolution of pharyngeal slits, which probably evolved
from “extensions” of the mouth. Pharyngeal slits allowed a larger surface area with
which to capture food.
Before jaws evolved, a body stiffener evolved to allow the larger protochordates
to become more active and seek out richer patches of food. This stiffener was the
notochord, which ran dorsally along the body and prevented telescoping of the body
when sinusoidal body movement occurred to propel the animal through it’s environment.
The muscle needed an area to work against the environment to produce force and
therefore movement, and so the region of the body posterior to the anus lengthened to
become the post-anal tail. The tail, powered by muscles attatched to the notochord,
swept back and forth to push the animal through the water to find food and escape from
predators.
As the tail and muscles evolved, a system was needed to control them - the
nervous system. A dorsal hollow nerve cord evolved that ran dorsally along the body,
dorsal to the notochord. Sensory structures began to form at the anterior region of the
animal, because that is the region that met the new environment first propelled by the tail.
A brain formed as these sensory structures collected formation that was integrated. The
brain sent messages down the spinal cord to the rest of the body, controlling its reactions
to the environment.
Now, we have a chordate with all 4 chordate features: notochord, post-anal tail,
pharyngeal slits, and dorsal hollow nerve cord. These 4 characteristics did not evolve
seperately; they co-evolved. Each one structure affected all the others.
However, we do not yet have a vertebrate.
The vertebral column initially formed from cartilage in order to protect the dorsal
hollow nerve cord and the dorsal major blood vessel. It was (is) segmentally arranged,
with neural and hemal arches. Initially it was made of cartilage, and the notochord was
the centra. As creatures grew larger and faster, and needed better body stiffening to
attatch muscle to and to work against. The vertebrae formed centra, which began to
squeeze out the notochord, which became reduced to intervetebral bodies in most higher
vertebrates.
See Fig. 5.
Jaws evolved after the vertebral colum did – after the lampreys, which are jawless
vertebrates. Gills evolved from pharyngeal slots during this time as well (see #2 for
detail).
Fig. 5: Development of vertebral column.
Answer #3 - assigned a score of 17/20:
The chordates arose within the deuterostomes (celomates in which the blastopore
develops into the anus in the embryo). According to the current model of evolution, a
deuterostome ancester first evolved a dorsal hollow nerve cord and pharyngeal slits; both
of these were lost in the echinoderms, but were retained in the hemichordates (acorn
worms). Chordate ancestors then evolved the notochord and postanal tail, and true
chordates with all 4 chordate characteristics appeared. One lineage of chordates became
the urochondates (tunicates). Another acquired somites – body segmentation – to
become the cephalochordates. A cephalocordate-like ancestor developed neural crest
cells, increased cephalication (true head), hypomere, and pronephric kidney, and became
a vertebrate. According to Garstang, chordate evolution occurred largely through
paedomorphesis – the retention of larval characteristics in a sexually mature adult. Some
deuterostomes (echinoderms) are sessile and quite unlike vertebrates as adults, but their
larvae are mobile and more similar to higher chordates, so maybe chordate evolution
advanced through elimination of the sessile adult and enhancement of the mobile larval
stage. In the evolution of vertebrates, the trend is toward increased size, mobility, and
activity; thus, the notochord gives support to the body and prevents “telescoping” so that
muscle contractions are translated into body movements, and a muscular tail provides
propulsion. Active lifestyle requires good nervous control of muscles and sensory
organs, so the nerve cord becomes more developed, with increased cephalication
(concentration of nervous/sensory organs at anterior end of animal); a cranium develops
to protect these sensory and nervous organs, and a true head develops. this is the
characteristic of a vertebrate. Later, vertebrae and hypermoticity of the body (lower
solute concentration in the body fluids than in the environment) appear.
Answer #4 - assigned a score of 18/20:
The 4 basic chordate characteristics, the 1) pharyngeal slits 2) notochord 3) post-anal tail
and 4) dorsal tubular nerve cord evolved to assist changes as organisms evolved from
sessile, filterfeeding lifestyle, to an active, predatory lifestyle. Ancestors to chordates
were sessile, filter feeders, using their pharyngeal slits to capture food. However, to
make the change to an active lifestyle, adaptations had to occur. The dorsal tubular nerve
cord contributed increased nervous control and sensory perception. The notocord
contributed to body stiffness which greatly increased the efficiency of body movement.
The post-anal tail provideded increased surface area that muscles could act on, these
muscles could make movement stronger and more efficient. The active vertebrate adult
has increased locomotory ability as compared to the sessile adults of all “prevertebrates”. However, these changes did not occur all at once, but rather over
evolutionary history. The current theory of vertebrate evolution as presented in Kardong
suggests that hemichordates and echinoderms branched together. The common ancestor
exhibited pharyngeal slits and a dorsal tubular nerve cord and these are present in
hemichordates, but secondarily lost in echinoderms. Urochordates evolved the other 2
chordate characteristics (post-anal tail and notochord) and thus are true chordates.
However, urochrodates possess these characteristics mainly as preadaptions, for although
we have seen that the chordate characteristics contribute to an active lifestyle, the tunicate
is a sessile filter feeder with a very simple nerve net type nervous system and no muscles.
Mesodermal somites, and thus segmented muscle evolved in the cephalochordates, such
as Amphioxus. Further important developments ocurred in the evolution of the
vertebrates such as increased complexity of the nervous system, skeletal system
(vertebral column, cranium) and organs (kidney). All evolutionary steps taken to create
the first vertebrates were focussed on the movement from sessile to active. This lifestyle
change required many elements of body structure to change and many advancements to
occur. The 4 chordate characteristics form the primary basis for these changes but were
combined with further advancements to form the active, predatory vertebrates we see
today.
Grading Key:
The origin of the vertebrates is still unsettled and controversial. Their origin
certainly lies somewhere amongst the invertebrates but the question is where?
Within the early chordates, the basic body plan was established consisting of
pharyngeal slits, a notochord, a dorsal hollow nerve cord and a post-anal tail.
The pharyngeal slits in extant vertebrates serve to separate suspended food
particles from the water as well as to extract oxygen and excrete carbon dioxide. Oxygen
is required to oxidize food substances to obtain ATP from metabolism and carbon
dioxide is the primary waste product of this process. All three processes are related
(obtaining and metabolizing food and excreting waste).
As animals grow, filter feeding becomes replaced by active feeding which
requires locomotion. Effective locomotion requires large muscle mass and a coordinated
nervous system. It also requires a stiff body upon which to generate force and a large
surface (the postanal tail).
Thus the basic chordate body plan is seen as a response to increasing size and the
need for active predation.
At one time or another almost ever group of invertebrates has been cited as the
evolutionary source of the chordates. The most plausible theories are based on anatomy
and embryology. There are several theories that are based on similarities seen in
echinoderms (starfish, sea cucumbers and sea urchins), hemichordates (acorn worms),
urochordates, (tunicates or sea squirts) and cephalochordates (amphioxus). The strongest
support for all of these theories is that all are deuterostomes.
At the end of the last century, Garstang put forward the idea that chordates
evolved from echinoderms through the hemichordates (acorn worms) and urochordates
(tunicates) to cephalochordates and vertebrates. While the adults of these various phyla
are extremely different and share few if any structural similarities, an evolutionary
progression can be seen in their larvae. His theory was that selection acted on the larval
stages to produce new forms. In the case of the hemichordates and urochordates, the
"new' larvae ultimately gave rise to sessile adults that were very different. Ultimately,
however, through paedomorphosis (or the retention of larval traits into sexual maturity)
they gave rise to the cephalochordates and vertebrates
The Dipleuruloid Theory was put forward by Malcolm Jollie, and is
similar to that put forward by Garstang with the primary distinction that rather than
maintaining that each group arose successively from the next, it maintains that each arose
from a common ancestor - a small cilliated, bilateral organism now extinct. He proposed
that this ancestor gave rise to the echinoderms, hemichordates and a prechordate that was
a predator with a differentiated head, a pharynx with gill slits, and a large mouth. The
urochordates and cephalochordates evolved such that their active larval form gave rise to
a sessile filter feeding life style while the vertebrates evolved towards an even more
active lifestyle.
In opposition to this, Gutmann proposed that the ancestral form of the chordates
were the cephalochordates (amphioxus like) and that the tunicates, hemichordates and
echinoderms are advanced forms all derived from this ancestor to fill specific niches. He
proposed that the vertebrates also arose from the cephalochordates along another
distinctive line
The most current theories now not only employ anatomical and embryological
data but also employ new molecular data obtained from DNA analysis. This data would
suggest that the ancestor of the vertebrates first gave rise to two groups. One group
ultimately gave rise to both the echinoderms and hemichordates while the other group
gave rise to the urochordates, cephalochordates and vertebrates. Except for details, this
theory does not differ significantly from the Dipleuruloid theory put forward by Jollie.
Remember, however, that even these theories remain controversial.
Thus, vertebrate evolution has tended to progress from sessile groups to active groups;
from filter feeding to active feeding involving muscular action and from soft body
support to bony support. This appears to have occurred through paedogenesis and lead to
increased locomotion and activity.